Page 1 of 38 Geological Magazine 1 2 3 1 Original Articles 4 5 6 2 Large trilobites in a stress-free Early Ordovician environment 7 8 3 FARID SALEH1*, MURIEL VIDAL2, LUKÁŠ LAIBL3,4,5, PIERRE SANSJOFRE2, PIERRE 9 10 4 GUERIAU3, FRANCESC PEREZ PERIS3, LORENZO LUSTRI3, VICTOIRE LUCAS2, 11 12 5 BERTRAND LEFEBVRE1, BERNARD PITTET1, KHADIJA EL HARIRI6 and ALLISON 13 14 3 15 6 C. DALEY 16 17 7 1Université de Lyon, Université Claude Bernard Lyon1, École Normale Supérieure de Lyon, 18 19 8 CNRS, UMR5276, LGL-TPE, Villeurbanne, France 20 21 For Peer Review 2 22 9 Univ. Brest, CNRS, IUEM Institut Universitaire Européen de la Mer, UMR 6538 Laboratoire 23 24 10 Géosciences Océan, Place Nicolas Copernic, 29280 Plouzané, France 25 26 11 3Institute of Earth Sciences, University of Lausanne, Géopolis, CH-1015 Lausanne, 27 28 12 Switzerland 29 30 31 13 4The Czech Academy of Sciences, Institute of Geology, Rozvojová 269, 165 00 Prague 6, 32 33 14 Czech Republic 34 35 15 5Institute of Geology and Palaeontology, Faculty of Science, Charles University, Albertov 6, 36 37 38 16 Prague, 12843, Czech Republic 39 40 17 6Département des Sciences de la Terre, Faculté des Sciences et Techniques, Université Cadi- 41 42 18 Ayyad, BP 549, 40000 Marrakesh, Morocco 43 44 * 45 19 [email protected] 46 47 20 Short running title: Large Ordovician trilobites 48 49 50 51 52 53 54 55 56 57 58 59 60 1 Cambridge University Press Geological Magazine Page 2 of 38 1 2 3 22 Abstract 4 5 6 23 Understanding variations in body-size is essential for deciphering the response of an organism 7 8 24 to its surrounding environmental conditions and its ecological adaptations. In modern 9 10 25 environments, large marine animals are mostly found in cold waters. However, numerous 11 12 26 parameters can influence body size variations other than temperatures, such as oxygenation, 13 14 15 27 nutrient availability, predation, or physical disturbances by storms. Here, we investigate 16 17 28 trilobite size variations in the Lower Ordovician Fezouata Shale deposited in a cold water 18 19 29 environment. Trilobite assemblages dominated by small- to normal-sized specimens that are 20 21 For Peer Review 22 30 few cm in length are found in proximal and intermediate settings, while those comprising 23 24 31 larger taxa more than 20cm in length are found in the most distal environment of the Fezouata 25 26 32 Shale. Drill core material from distal settings shows that sedimentary rocks hosting large 27 28 33 trilobites preserved in-situ are extensively bioturbated with a high diversity of trace fossils, 29 30 31 34 indicating that oxygen and nutrients were available in this environment. In intermediate and 32 33 35 shallow settings, bioturbation is less extensive and shallower in depth. The rarity of storm 34 35 36 events (minimal physical disturbance) and the lack of predators in deep environments in 36 37 38 37 comparison to shallower settings would have also helped trilobites attain larger body sizes. 39 40 38 This highly resolved spatial study investigating the effects of numerous biotic and abiotic 41 42 39 parameters on body size has wider implications for the understanding of size fluctuations over 43 44 45 40 geological time. 46 47 41 Keywords: Arthropod, Body-size, Paleozoic, Fezouata Shale 48 49 50 51 52 53 54 55 56 57 58 59 60 2 Cambridge University Press Page 3 of 38 Geological Magazine 1 2 3 43 1. Introduction 4 5 6 44 Considered one of the most important aspects of animal biology (Bonner, 2006), body size 7 8 9 45 results from numerous biotic and abiotic factors (Bell, 2014). Vertebrate size variations over 10 11 46 geological time have received considerable attention (Sander & Clauss, 2008; Geiger et al., 12 13 47 2013). Comparatively, marine invertebrates have been less studied (Lamsdell & Braddy, 14 15 16 48 2009; Klug et al., 2015; Sigurdsen & Hammer, 2016). For instance, it is well agreed that low 17 18 49 temperatures can be responsible of the large sizes of modern marine invertebrates (i.e. 19 20 50 Bergmann's rule; Timofeev, 2001; Moran & Woods, 2012). Nevertheless, if this was the sole 21 For Peer Review 22 51 parameter controlling body size, all taxa at high latitudes should be larger than genera found 23 24 25 52 at lower latitudes. This is rarely the case because size variations occur locally in a specific 26 27 53 paleoenvironment, owing to changes in water depth, oxygenation, predation, nutrient 28 29 54 availability or even physical disturbances caused by storm events (Saleh et al., 2018). 30 31 32 55 During the Ordovician, Morocco was part of the Gondwana margins, at high latitudes, close 33 34 56 to the South Pole. The Fezouata Shale was deposited near the Zagora region in Morocco, 35 36 57 under cold waters at the transition between two major evolutionary events: the Cambrian 37 38 39 58 Explosion and the Great Ordovician Biodiversification Event (Martin et al., 2016a). In this 40 41 59 formation, two sedimentary intervals have yielded thousands of exceptionally preserved 42 43 60 fossils belonging to different groups such as arthropods, echinoderms, mollusks, and sponges 44 45 61 (Vinther et al., 2008, 2017; Van Roy et al., 2010, 2015a; Martí Mus, 2016; Lefebvre et al., 46 47 48 62 2019). A striking feature of this formation is extreme body size fluctuations at both taxon and 49 50 63 assemblage scales between localities and even between different levels of the same locality 51 52 64 (for further details see Ebbestad, 2016; Lefebvre et al., 2016; Martin, 2016; Saleh et al., 53 54 55 65 2018). Trilobites occur in all sites from the Fezouata Shale and show a large body-size range 56 57 66 in this formation. Abundant and spectacular specimens of very large trilobites were found at 58 59 67 Ouled Slimane near the Tanskhit bridge (Rábano, 1990; Fortey, 2009; Lebrun, 2018). In this 60 3 Cambridge University Press Geological Magazine Page 4 of 38 1 2 3 68 study, the sedimentological and taphonomic contexts of levels with large trilobites from the 4 5 6 69 Fezouata Shale are elucidated, in order to contribute to the understanding of body-size 7 8 70 fluctuations in the geological record (see also Lamsdell & Braddy, 2009; Klug et al., 2015; 9 10 71 Sigurdsen & Hammer, 2016). 11 12 72 13 14 15 73 2. Geological and paleoenvironmental context 16 17 18 74 A long term transgression at the beginning of the Ordovician created epicontinental seas on 19 20 75 the Gondwana margins in the Southern Hemisphere (Torsvik & Cocks, 2011; 2013). The 21 For Peer Review 22 76 Fezouata Shale Formation (Fig. 1a) was deposited in a cold water sea, at high latitudes (over 23 24 25 77 60°S) close to the South Pole (Fig. 1b) (Torsvik & Cocks, 2013; Martin et al., 2016b). 26 27 78 Sedimentary rocks of this formation consist of blue-green to yellow-green siltstones 28 29 79 (Destombes et al., 1985). The 900 m-thick succession of the Fezouata Shale (Fig. 1a) was 30 31 32 80 deposited in a storm/wave dominated environment with a minor influence of tides (Vaucher et 33 34 81 al., 2016). In this environment, sedimentological structures indicate a deepening trend from 35 36 82 the SE to the NW (Fig. 1c) as shown in Vaucher et al. (2017). Thus, the most proximal 37 38 39 83 settings in the Late Tremadocian (A. murrayi Zone) occur near Tamegroute (about 20 km 40 41 84 ESE of Zagora; Fig. 1c) (see, Saleh et al., 2018). In this locality, sedimentary rocks are 42 43 85 constituted of coarse siltstones to fine grained sandstones showing hummocky cross 44 45 86 stratifications (HCS) of cm- to dm-scale wavelengths (Vaucher et al., 2016) (Fig. 1d). 46 47 48 87 Intermediate settings of the Fezouata Shale occur in Bou Izargane in the Ternata plain about 49 50 88 20 km N of Zagora (Fig. 1c). In this setting, sedimentary rocks are characterized by finer 51 52 89 siltstones and more abundant background sediments than in Tamegroute, in addition to the 53 54 55 90 presence of storm events with up to cm-scale HCS (Saleh et al., 2019) (Fig. 1d). The average 56 57 91 sedimentation rate in this area was estimated ~79 m/Ma (Saleh et al., 2019). The progradation 58 59 92 model proposed by Vaucher et al., (2017) suggests that the Ouled Slimane area is associated 60 4 Cambridge University Press Page 5 of 38 Geological Magazine 1 2 3 93 with more distal settings (Fig. 1c). A field campaign was organized in 2019 to better constrain 4 5 6 94 the depositional environment of this locality (see section 3). 7 8 95 As for the faunal content of the different sites, all three sites yielded diverse assemblages of 9 10 96 marine invertebrates (Saleh et al., 2018). However, Tamegroute is characterized by sessile 11 12 97 epibenthic taxa (bivalves, brachiopods) that are about half the size of those in Bou Izargane 13 14 15 98 (Saleh et al., 2018). Size variations in the Fezouata Shale between localities are not limited to 16 17 99 brachiopods and bivalves. Trilobites also show body-size discrepancies between localities. 18 19 100 The largest trilobites from the Fezouata Shale are found at Ouled Slimane (Rábano, 1990; 20 21 For Peer Review 22 101 Fortey, 2009; Lebrun, 2018). 23 24 102 <insert figure 1> 25 26 103 3. Material and methods 27 28 29 104 Two successive field campaigns were carried out in the Zagora region in 2018 and 2019 and 30 31 32 105 two cores were obtained.